Coexistence of two D-lactate-utilizing systems in Pseudomonas putida KT2440

It is advantageous for rhizosphere-dwelling microorganisms to utilize organic acids such as lactate. Pseudomonas putida KT2440 is one of the most widely studied rhizosphere-dwelling model organisms. The P. putida KT2440 genome contains an NAD-dependent D-lactate dehydrogenase encoding gene, but mutation of this gene does not play a role in D-lactate utilization. Instead, it was found that D-lactate utilization in P. putida KT2440 proceeds via a multidomain NAD-independent D-lactate dehydrogenase with a C-terminal domain containing several Fe-S cluster-binding motifs (Fe-S D-iLDH) and glycolate oxidase, which is widely distributed in various microorganisms. Both Fe-S D-iLDH and glycolate oxidase were identified to be membrane-bound proteins. Neither Fe-S D-iLDH nor glycolate oxidase is constitutively expressed but both of them can be induced by either enantiomer of lactate in P. putida KT2440. This study shows a case in which an environmental microbe contains two types of enzymes specific for D-lactate utilization.